This invention relates to a magnetic recording medium and, more particularly, to an evaporated type magnetic recording medium.
In the field of a video tape recorder etc., there is recently raised a strong demand for high density recording for achieving high picture quality. In keeping up therewith, there is proposed a thin magnetic metal film type magnetic recording medium in which a magnetic metal material or a magnetic alloy material, such as CoNi, is directly deposited on a non-magnetic substrate by plating or vacuum thin film forming technique, such as vacuum deposition, sputtering or ion plating.
The thin magnetic metal film type magnetic recording medium has a number of advantages, such that it is superior in coercivity, rectangular ratio and electro-magnetic conversion characteristics in a short wavelength range and lends itself to reduction in thickness of the magnetic layer and to increased packing density of the magnetic material since there is no necessity of mixing the non-magnetic material, such as a binder, in a magnetic layer, as in the case of a coated magnetic recording medium.
In particular, the evaporated type magnetic tape, or the evaporated tape, which has its magnetic layer formed by the vacuum deposition method, is commercialized as a high-band 8 mm tape or a digital micro-tape, such as a non-tracking (NT) tape, since it has high production efficiency and stable characteristics.
In the above-described evaporated tape, the magnetic layer is formed by vacuum deposition which is carried out in such a manner that a magnetic material as a source of evaporation is irradiated under vacuum with electronic rays so as to be converted into metal vapor which is then deposited on the non-magnetic substrate. In this case, the general practice is to introduce an oxygen gas into the atmosphere of vapor deposition for oxidizing part of the metal vapor for controlling magnetic characteristics, such as coercivity or saturation magnetic flux density of the magnetic layer.
FIG. 1 shows the constitution of a vapor deposition apparatus in which vapor deposition is carried out while introducing oxygen into the vapor deposition atmosphere.
The vapor deposition apparatus includes a vacuum chamber 30 maintained at high vacuum and accommodating a feed roll 31 and a take-up roll 32 therein. A tape-shaped non-magnetic substrate 33 is adapted for travelling from the feed roll 31 to the take-up roll 32.
At a mid portion of a travel path of the non-magnetic substrate 33 from the feed roll towards the take-up roll 32 is mounted a cooling can 34 larger in diameter than the rolls 31, 32. The cooling can 34 is positioned for pulling the non-magnetic substrate 33 towards below in FIG. 1.
Thus the non-magnetic substrate 33 is sequentially reeled out from the feed roll 31 so as to be guided around the peripheral surface of the cooling can 34 before being taken up on the take-up roll 32.
Within the vacuum chamber 30, a crucible 35 filled with a magnetic metal material 38 as a source of evaporation is mounted below the cooling can 34. On the inner side wall of the vacuum chamber 30 is mounted an electron gun 16 for heating and evaporating the magnetic metal material 38 filled in the crucible 35. The electron gun is oriented for radiating electron rays therefrom to the magnetic metal material 38 contained n the crucible 35. The magnetic metal material 38, evaporated by the electron gun, is deposited as a magnetic layer on the non-magnetic substrate 33 adapted for running at a constant velocity around the peripheral surface of the cooling can 34.
Between the cooling can 34 and the crucible 35, there is mounted a shutter in the vicinity of the cooling can 34. The shutter 36 is formed for covering a pre-set area of the non-magnetic substrate 33 adapted for travelling on the peripheral surface of the cooling can 34. The magnetic metal material 38, evaporated by the shutter 36, is deposited obliquely within a pre-set angular extent on the non-magnetic substrate 33.
With the present vacuum deposition apparatus, an oxygen gas inlet 37 is provided through the sidewall section of the vacuum chamber 30. The oxygen gas inlet 37 is provided in close proximity to the cooling can 34 between the shutter 36 and the cooling can 34, that is at a position spaced apart from the magnetic metal material 38 operating as an evaporation source.
Part of the metal vapor from the vaporization source and travelling past the shutter is oxidized by the oxygen gas supplied via the oxygen gas inlet 37 so as to be deposited on the surface of the non-magnetic substrate 33.
The thin magnetic metal film, formed by the above-described oxidation process, tends to be higher in coercivity and lower in saturation magnetization than the thin magnetic metal film which has not passed through the oxidation process. Thus the magnetic properties may be adjusted by controlling the amount of oxygen introduced via the oxygen gas inlet.
The oxygen inlet is provided between the shutter and the cooling drum in order to prevent the oxygen gas inlet from being directly exposed to the metal vapor from the vaporization source by the shutter. If the oxygen gas inlet is directly exposed to the metal vapor, the latter tends to be affixed to the oxygen gas inlet thus obstructing normal introduction of the oxygen gas.
In view of uniforming the quantity of the introduced oxygen gas in a direction along the width of the non-magnetic substrate, a number of inlet tubes of small diameters are placed in parallel along the width of the non-magnetic substrate, or an inlet tube having a uniform cross-sectional shape along the width of the non-magnetic substrate is employed.
If the thin magnetic film formed by the above-described vacuum deposition apparatus is analyzed as to its oxygen concentration by e.g., the Auger electron spectroscopic method, it is found that the degree of oxidation in the thin film is not uniform but is significantly partialized towards its surface layer. In the case of a thin magnetic metal film having a thickness of 200 nm, its surface layer with a thickness ranging from 16 to 28 nm represents a surface oxide layer exhibiting an extremely high oxidation degree. Heretofore, the presence of the surface oxide layer was felt to raise magnetic characteristics, such as coercivity, of the magnetic layer, and to improve its weatherability.
However, the results of our investigations have revealed that the surface oxide layer does not necessarily contribute to improving magnetic properties, such as coercivity, but may impair electro-magnetic conversion characteristics, because the magnetic properties have been deteriorated in such surface layer.